The goal of this project is to characterize voltage-dependent ion channels of neurons, and their regulation by neurotransmitters. Both frog sympathetic neurons and rat thalamic neurons will be studied, and the properties of voltage-dependent calcium channels will be emphasized. Whole-cell and single-channel patch damp methods will be used. Long-term objectives of this work are to clarify how the detailed properties of specific types of ion channels contribute to the overall electrical behavior of the cell, and to the cell's role in neuronal circuits and behavior. One long-term goal of the work on thalamic neurons is to determine the ionic mechanisms underlying absence seizures (petit mal epilepsy). Four specific aims relate to frog sympathetic neurons. (1) The properties of a newly recognized calcium current will be further characterized. Specific experiments include the effect of divalent cations (Ca2+ and Ba2+) on current through this channel, and the effects of cone snail and spider toxins. One goal is to determine whether this channel was previously misidentified as the basis of the whole-cell omega- conotoxin-sensitive N-current. (2) The voltage- and Ca2+-dependence of inactivation of the N-type calcium current will be examined. The increased inactivation caused by inhibition of protein phosphatases will be examined at the single-channel level. (3) Modulation of calcium channels by neurotransmitters and G proteins will be examined at the single-channel level. The biochemical mechanisms of modulation of calcium current, and of the M-type potassium current, will be compared. (4) The mechanisms of ion permeation and block of calcium channels will be characterized. A fifth specific aim relates to neurons acutely isolated from the ventrobasal nucleus of the thalamus of neonatal rats. (S) Kinetic and pharmacological criteria will be used to determine which types of high-voltage-activated calcium channels are present in thalamic neurons. The regulation of thalamic calcium channels by neurotransmitters will be investigated. These studies will contribute to our understanding of the diversity of neuronal calcium channels, and their functions in neuronal excitability.